Broad-band communication networks make it possible to exchange the most diverse kinds of digital data by means of the Asynchronous Transfer Mode (ATM) in accordance with a CCITT recommendation. A logical connection, which is established for a short time, occupies a so-called virtual channel. The data flow, i.e. the mount of data passing through this channel per unit of time, depends on the type of communication (speech, moving real-time images, data exchange, etc.) and occupies a corresponding portion of the total data flow that is available in the physical transmission medium. For example, voice transmission requires very few constant data units per unit of time, while a data exchange preferably uses many data units per unit of time, but without special requirements regarding the time sequence. The data flow in a virtual channel can increase and decrease and be fully exhausted in the course of time. The data flow can be correctly estimated for many types of transmissions, such as perhaps video transmissions.
The required capacity in the transmission medium can be kept available per virtual channel or, with several channels held in common in a certain network section, per virtual path in accordance with an agreement. In that case, no doubt the most important parameter is the number of data units that can be transmitted per unit of time, the so-called bit rate. Beyond that, an agreement between network operator and customer can address further parmeters, such as perhaps a short time during which a certain number of data units can follow each other more rapidly. Maintenance of such agreements at the customer-network interface is provided by a special circuit part, the usage parameter control (UPC). It is often installed at the input of an ATM switching unit. If the agreement is not complied with, the UPC rejects any excess data units.
Although the agreements arrived at in this manner prevent an overload, namely of the switching unit, they also result in that the potential capacity of the connections remains unused. It is therefore intended to offer a broad-band network service which permits the utilization of the available bit rate frequency. A customer who utilizes this service can make data transfers up to the maximum load of the connection, beyond the agreed capacity that is always available to him, where the maximum can be determined by another agreement or by the highest bit rate permitted by the circuit technology. This service is of particular interest to the exchange of data that is not critical with respect to time.
On the other hand, this expansion of the connection load produces the danger of overloading the switching unit. A possibility of reducing this danger consists in providing a buffer for each physically available connection, which altogether limits the data flow through this connection. However, it can happen that the data flows are relatively heavily restricted in the paths passing through this connection, even though the switching unit is not yet overloaded by a wide margin.